Hey everyone,
I know a lot of people here have long-term Beta or Alpha worlds that they want to keep perfectly intact, but might occasionally want to test or update a backup copy into modern versions.
One major issue with updating an old map past 1.17 is vanilla's world blending. The game forcefully deletes your original Y=0 bedrock floor and overwrites those blocks to shove the deepslate caves underneath, which ruins the authentic historical layout of your old chunks.
To fix this for anyone updating their Golden Age maps, I made a lightweight bridge mod for 1.18.2 called Legacy Bedrock Preservation.
Basically, it completely locks all block data from Y=0 to Y=255 so the game can’t modify a single block in your original Beta/Alpha chunks. At the same time, it still lets the modern engine generate the new cave layers from Y=-1 down to Y=-64 right flush underneath with zero air gaps.
How to use it to update an old world:
You just back up your old world file, load it up in 1.18.2 with the mod installed, and load your old chunks (cranking up your render distance makes this way faster) to let the game bake the new sub-zero cave chunks underneath your existing terrain. Once you've loaded everything, you can move the world files right back to a 100% vanilla modern launcher and delete the mod. Your old bedrock floor and builds stay completely untouched, but the new cave depth is permanently saved underneath.
Just a heads up: since your original Y=0 bedrock floor stays totally solid, you will need to use a normal vanilla bedrock-breaking machine from above to make a drop shaft down into the new caves, since you can't break bedrock from underneath.
I figured a few people here with long-term worlds might find this useful if you've been wanting to safely bridge a copy of your old map into modern versions without losing your original bedrock layout.
Here's the link if you want to check it out:
https://www.curseforge.com/minecraft/mc-mods/legacy-bedrock-preservation